1. Field of the Invention
The present invention generally relates to an apparatus for removing dissolved gasses from liquid metals. More specifically, the present invention relates to an inline Multi-Mode Gas Activation (MGA) degassing apparatus which may be used to remove dissolved hydrogen from liquid aluminum or aluminum alloys as part of a continuous or semi-continuous casting operation. The invention is particularly useful on aluminum alloys which are required to be cast in ultraclean conditions and on alloys which inherently have high solubility of hydrogen, such as aluminum-lithium alloys and aluminum-magnesium alloys.
2. Description of Related Art
Metal manufacturing and casting, such as aluminum manufacturing and casting, involves a considerable number of different components. Generally, a casting line will have a furnace that heats aluminum into a liquid and various casting assemblies that can be used to create particular shaped aluminum pieces. Casting lines will also typically have various components that are directed towards purifying the metal into a desired level of purity for a particular application.
For example, high-purity aluminum and aluminum alloys are used for applications in a large number of industries, such as pharmaceutical, semiconductor, foil, and aerospace applications. In these applications, the smaller the amount of impurities, the higher is the uniformity of the material. In some applications, high-purity aluminum and aluminum alloys are characterized by ultra low hydrogen content, in the range of less than 0.07 cc/100 gm Al, and further by exceptionally low impurity content of inclusions in the range of 0.005 mm2/kg.
To obtain high-purity aluminum and aluminum alloys a variety of different degassing techniques can be employed. In one technique, liquid aluminum containing impurities is exposed to Argon gas that bubbles through the liquid aluminum and engages with the impurities causing the impurities to be captured on the gas bubble and then removed from the aluminum melt. This technique is suitable for small and large volume aluminum casting lines; lines that are producing, for example, from 50 lbs to over 500 lbs of aluminum per minute. The art of degassing with argon and halogen gas is described in U.S. Pat. Nos. 3,645,520; 3,743,263; 5,234,202; 5,364,078; and 5,846,481.
However, many difficulties occur with such processing lines. First is that Argon is an expensive material and this process requires continuous use of this gas. This process also requires sophisticated gas delivery and recovery systems which can also add to the cost of producing high purity aluminum.
The practice and technique of degassing aluminum with vacuum is given in: U.S. Pat. Nos. 5,415,680; 3,895,937; 2,809,107; 5,167,698; 4,378,242; 4,014,529; 2002/0121158A1; U.S. Pat. No. 7,666,248B2; Chinese patents 101985698, 201873727, 201665699; WO8606749A1; Russian patent 2361938. The practice of applying ultrasonic energy for the degassing purpose is disclosed in U.S. Pat. Nos. 7,682,556; 4,287,755; 4,235,627; 4,662,215; 4,770,699; 4,261,197; 6,736,010B1; 7,021,145B2; 4,763,513; 4,373,950; 6,429,575B1; 6,244,738B1; 5,803,106; 2001/0,2247,456; 3,434,823.
The application of ultrasonic methods for degassing metals has so far been tried only on stagnant metals in batch processes and not on continuously moving metal systems. The stagnant melts are quickly prone to generation of “standing waves” in the molten metal batch and effectively a very low volume of the melt is degassed even though the energy is applied to the entire melt. Therefore, the use of in-situ cavitation method to enhance the diffusivity of hydrogen atoms so that they will rapidly transport themselves and nucleate as hydrogen molecules for their effective removal from the melt, requires simultaneous existence of mechanized fluid flow which the present art fails to provide.
Other ways to purify aluminum or aluminum alloys, which involve using low pressure systems that draw the hydrogen gas out of the liquid aluminum, are typically limited to small batch operations that use such low pressure vacuum systems. These small-batch vacuum systems consist of a small vacuum chamber where liquid aluminum is introduced in small batch quantities ranging from 5 to 10 pounds. Although this method is able to achieve sufficient quality, the amounts produced in small batches and the resources needed to produce these small batches makes this a very costly and inefficient method of manufacturing high-purity aluminum in larger tonnages or in a continuous manner. Consequently, these types of systems are generally not used for high volume aluminum production lines. The method of vacuum degassing large tonnages of Aluminum from 1 ton to 20 tons is practiced today in Russia, however, the cost of the furnace is extremely high and the benefit is lost during the transit as the aluminum is transported from vacuum furnace to casting station when it regasses itself due to atmospheric humidity.
Hence, there is a need for an improved process and system for purifying metals and, in particular, purifying aluminum and its alloys in such a way that the purification is carried out “in-line” and close to the casting system in a continuous manner. Additionally, there is a need for a process and system that removes impurities such as hydrogen without requiring the introduction of large quantities of expensive gases and the accompanying equipment to safely deal with these gases. In addition, there is a need to achieve degassing values close to the thermodynamically possible values by simultaneously providing other means of activating the diffusion of hydrogen atoms.